Modern semiconductor fabrication facilities (known as “fabs”) use tools that can process multiple wafers. The wafers are typically delivered to a tool from a stocker. Currently semiconductor FABS are often not limited by tool performance, but due to the availability of front opening unified pods (FOUP) at the tools equipment front end module (EFEM). This can be due to several causes including not enough work in process (WIP) at the tool and insufficient transport capability or scheduling flexibility in the Automated Material Handling System (AMHS) or Overhead Track System (OHT). If a tool has all its load ports loaded the AMHS must remove a finished FOUP prior to dropping off a new FOUP for that location. This requires the AMHS or OHT to make multiple trips to the tool from the stocker. Utilizing localized storage can reduce this impact but not eliminate it.
There have been several attempts to deal with this problem. For example, often tools will be ordered with three or four load ports instead of two to try and reduce the burden on the AMHS/OHT during peak periods. This allows the tool to work on two FOUPS while the other load ports are being loaded or unloaded by the AMHS. Unfortunately, this solution is less than optimal due to increased size cost and complexity of the EFEM, larger fan filter unit and increased overall tool footprint in the FAB. In addition, it does not always reduce the number of moves required by unidirectional OHT cars. If the FOUP to be retrieved is not downstream from the drop-off locations, an additional car or an additional circuit of the OHT must be performed.
Furthermore, in the present state of the art, even if two OHT cars work in parallel, processing of FOUPs could still take longer due to scheduling delays. Most scheduling systems will not dispatch a car to retrieve a new FOUP from storage until the finished FOUP has been removed from the load port. For example, in a typical mode of operation, a first OHT car is dispatched to a tool that has completed processing a FOUP. The first car collects the finished FOUP and delivers it to storage or another tool, after which the first car is free for the next task. After the first car has picked up the finished FOUP, the tool is available for a new FOUP and a second car is dispatched with a new FOUP. The second car unloads the FOUP at a load port of the tool. The second car is then available for the next task.
Other prior art solutions have attempted to provide some sort of local storage for FOUPs at or near the tool. For example, the Brooks OneFab™ AMHS provide storages e.g., shelves, in area under the OHT near the tool to store one or more FOUPs until the tool or AMHS/OHT becomes available. Although this reduces the time to move the FOUP from the Stocker it does not reduce the burden on the OHT. For example, the OHT must move the FOUPs between the tool front end and the shelves. Furthermore, FOUPs stored on the shelves are not easily accessible for manual hot lots or if the OHT goes down. A further drawback is that the AMHS must know in advance which tool and will be ready next. If the FOUP is stored at localized storage that is in another bay or area of the FAB significant delay may be incurred due to the added required travel. In extreme cases this could require handling by both intrabay and interbay OHT.
Another prior art approach has been to integrate a mini stocker into the tool front end. This approach allows several FOUPs to be stored onto the tool. The mini stocker typically includes one or more drop-off locations and a gantry robot for moving the FOUPs to and from the load ports on the tool front end. Mini-stocker systems can be dedicated to one tool as in Applied Materials Bay Distributed Stocker®, or span several tools as in Asyst's FasTrack®. For systems of this type, the OHT delivers FOUPS to the stocker, and the stocker transports the FOUPS to the load port. In the case of the Bay Distributed Stocker®, an integrated Gantry stores and places FOUPs on load ports that are mounted to the tool. Integrated mini-stockers are both expensive and complex. They also present a single point of failure for the system and do not readily allow for manual removal of FOUPs. Furthermore, an integrated mini-stocker can take up additional footprint on the floor of the fab. This is particularly disadvantageous in manufacturing environments where floor space is at a premium.
Another prior art approach is to add a standalone stocker to the EFEM. An example of such a system is available from Vertical Solutions. This type of system uses a form of gantry to place and retrieve FOUPs directly from a single load port. The OHT delivers FOUPS to the stocker, and the stocker transports the FOUPS to the load port. Like the integrated mini-stocker, the standalone stocker is both expensive, complex and a single point of failure. In addition, standalone stockers do not readily allow for manual removal of the FOUPs. The standalone stocker also takes up additional footprint on the floor of the fab. Furthermore, standalone stockers do not effectively reduce the burden on the OHT system. The AMHS would need prior knowledge of what tool or bay would have an available load port next. Current tools only signal completion. They do not inform the AMHS that they are on the last wafer, or a significant percent complete.
Thus, there is a need in the art, for a method for reducing the load on an automated material handling system during processing of materials that overcomes these disadvantages and an apparatus for implementing such a method.